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Suppose my program prints to multiple file descriptors and I need to watch all of the outputs as they are being printed but only save stdout to a file. How do I do that?

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Suppose 3 is one of those file descriptors you're interested in. 3 may be pointing to file, for example:

exec 3>file

Your app writes to 3:

app() { echo hello world >&3; }

If you want to interecept 3, you need to dup it to another file descriptor (e.g., 4):

exec 4>&3

and redirect (=replace) 3 with a pipe to your interceptor process, which has to write back to the original target (or else it wouldn't be an interceptor), which is now saved in fd 4:

app 3> >(tee /dev/tty >&4) #this interceptor writes to the terminal

The above should print hello world to the terminal and file should end up with hello world as well.

All this assume these filedescriptors are open before you start the process. If you want to intercept filedescriptor outputs dynamically at runtime, then I'm afraid you have to modify the code, inject code at runtime, or intercept system calls with something like ptrace.

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